In many optical printing systems the intensity of a laser light beam focused on a two-dimensional photosensitive surface is modulated as the beam is moved relative to such surface to provide a two-dimensional output image. Such systems often use an output scanner which may include a gas laser which produces a beam of light at a predetermined wavelength and a deflector such as a rotating polygon mirror which line scans this light beam. The intensity of this laser light beam is information modulated by an acoustooptic modulator device.
This type of modulator includes a transparent cell which may be made of an acoustooptic material such as glass or TeO.sub.2 crystal and a piezoelectric transducer bonded to the cell. An RF signal, usually in the range of 40-300 MHz, is applied to the transducer. The transducer launches acoustic waves in the cell which produces sonic compression waves that create a diffraction wave grating. This diffraction grating causes a portion of the input laser light beam passing through the cell to be diffracted out of its original path. Amplitude changes of the RF signal cause intensity modulation of the diffracted (first-order) and undiffracted (zero-order) beams. The intensity of the modulated diffracted light beam varies in direct proportion to RF signal amplitude. The modulated diffracted light beam, rather than the undiffracted beam, is utilized, e.g. applied to a deflector which converts the information modulated light beam into a line scan.
In some printing applications, it is desirable that noise variations in such input light beam intensity about a desired constant intensity level, be kept on a very low level. For example, with laser color printers there are applications where it is very important that the DC laser power variations at the image zone be kept at less than about .+-.0.5% from a desired level to prevent banding in prints.
In commonly assigned U.S. patent application Ser. No. 619,453 filed June 11, 1984 to Baldwin et al, a beam intensity controlling apparatus is disclosed which includes an acoustooptic cell which receives a noisy input laser light beam and produces a substantially constant zero-order beam and a first-order light beam in response to an input RF signal at a predetermined frequency. The first-order beam is varied to control the intensity of the zero-order beam. To this end, means respond to the intensity of the zero-order beam to produce an error signal which is a function of the difference in intensity of the zero-order beam from a desired constant intensity level. The difference in intensity of the zero-order beam from the desired constant level intensity beam has two components, a DC component which represents a slow or long term shift changes in the zero-order beam intensity and an AC component which represents faster time varying changes in the zero-order beam intensity. The error signal is provided to adjustable means which cause a change in the amplitude of the RF signal. This in turn changes the intensities of the zero-order and first-order beams so that the zero-order beam is adjusted towards the desired constant intensity. The zero-order beam is then applied as an input to a utilization device.
An advantage of applying the zero-order beam produced by an acoustooptic cell to the utilization device rather than the first-order beam is that the zero-order beam permits highly efficient throughput of laser energy since only a small amount of laser power needs to be diverted to the first-order beam to control beam intensity.
This arrangement provides a significant advance in the art. However, in certain applications it may have problems maintaining variations in the DC component of the output laser beam at the image zone within acceptable tolerances.